Propionic acid is a material which can be used as solvent, as food preservative or in the herbicide manufacture; propionic acid also participates in the preparation of vinyl propionate, which is used as monomer in (co)polymers with, for example, ethylene, vinyl chloride or (meth)acrylic esters.
Processes for the synthesis of propionic acid are known in the prior art. For example, patent application DE 102 25 339 A1 describes a process for the preparation of propionic acid by catalytic hydrogenation of acrylic acid in the presence of molecular oxygen and of a catalyst of an element from groups 8 to 11. Conventionally, acrylic acid is obtained by catalytic gas-phase oxidation of propane, propylene and/or acrolein.
One of the problems posed by the processes for the synthesis of propionic acid of the prior art is that they are carried out starting from nonrenewable starting materials of fossil (oil) origin, in particular propane or propylene. In point of fact, resources of these starting materials are limited and the extraction of oil requires drilling at increasingly deep depths and under technical conditions which are always more difficult, requiring sophisticated equipment and the use of processes which are always more expensive in energy. These constraints have a direct consequence with regard to the cost of manufacturing propionic acid.
Furthermore, manufacturers for some years have directed their research and development studies at “bioresourced” processes of synthesis using renewable natural starting materials.
For example, for the manufacture of acrylic acid resulting from renewable resources, alternative processes have recently been developed starting from nonfossil plant starting materials. In particular, processes starting from glycerol (also known as glycerin), resulting from the methanolysis of fatty substances, have been developed. This glycerol is available in large amounts and can be stored and transported without difficulty.
The methanolysis of vegetable oils or animal fats can be carried out according to various well known processes, in particular by using homogeneous catalysis, such as sodium hydroxide or sodium methoxide in solution in methanol, or by using heterogeneous catalysis. Reference may be made, on this subject, to the paper by D. Ballerini et al. in l'Actualité Chimique of November-December 2002.
As regards the conversion of glycerol by the chemical route, mention may be made of the synthesis of acrylic acid in two stages, namely the production of acrolein by dehydration of glycerol, which is described in particular in patent U.S. Pat. No. 5,387,720, followed by a “conventional” oxidation of the acrolein to produce acrylic acid.
The first stage in the manufacture of acrylic acid from glycerol results in the same intermediate compound as the conventional manufacturing process starting from propylene, namely acrolein, according to the reaction:CH2OH—CHOH—CH2OH→CH2═CH—CHO+2H2Owhich is followed by the second oxidation stage according to the reactionCH2═CH—CHO+½O2→CH2═CH—COOHPatent applications EP 1 710 227, WO2006/136336 and WO2006/092272 describe such processes for the synthesis of acrylic acid from glycerol comprising the stage of gas-phase dehydration in the presence of catalysts consisting of inorganic oxides (mixed or unmixed) based on aluminum, titanium, zirconium, vanadium, and the like, and the stage of gas-phase oxidation of the acrolein thus synthesized in the presence of catalysts based on oxides of iron, molybdenum, copper, and the like, alone or in combination in the form of mixed oxides.
However, one of the problems posed by these processes is that the acrylic acid is not the only product formed and that by-products are formed in large amounts, such as propionic acid and impurities, such as water, acrylic acid dimers, acetic acid, acrolein, benzaldehyde, furfurals or hydroquinone. It is thus generally necessary to purify the acrylic acid by conventional techniques in order to obtain a more concentrated acrylic acid solution.
As the quality of the acrylic acid, that is to say its content of various impurities, plays a large role in the subsequent polymerization processes, manufacturers manufacturing this acrylic acid have been led to bring into play a whole series of purification stages in order to obtain a “standard” acrylic acid, which is normally referred to as glacial acrylic acid (gAA). This gAA does not meet officially recognized specifications having a universal nature but means, for its manufacturer, the level of purity to be achieved in order to be able to successfully carry out its subsequent conversions. By way of example, for an acrylic acid resulting from propylene, the reactor outlet stream is subjected to a combination of stages which can differ in their sequence depending on the process: removal of the noncondensable products and of the bulk of the very light compounds, in particular the intermediate acrolein in the synthesis of the acrylic acid (crude AA), dehydration removing the water and the formaldehyde (dehydrated AA), removal of the light products (in particular the acetic acid), the removal of the heavy products, optionally removal of certain residual impurities by chemical treatment.
This process is highly analogous to the process for synthesis from propylene in so far as the intermediate product, the acrolein, resulting from the first stage is the same and as the second stage is carried out under the same operating conditions. However, the first-stage reaction of the process of the invention, dehydration reaction, is different from the propylene oxidation reaction of the normal process. The dehydration reaction, carried out in the gas phase, is carried out using solid catalysts different from those used for the oxidation of propylene. The aerolein-rich stream resulting from the first dehydration stage, intended to feed the second stage of oxidation in the acrolein to give acrylic acid, comprises a greater amount of water and in addition exhibits substantial differences as regards by-products resulting from the reaction mechanisms involved being given material formed by the different selectivities in each of the two routes.
In order to illustrate these differences, the data relating to the presence of various acids in the crude acrylic acid, that is to say in the liquid phase exiting from the reactor of the second stage according to the state of the art, are collated in table 1 below.
TABLE 1Impurity/AA (crude acrylic acid)Ex-propyleneEx-glycerolratio by weightprocessprocessAcetic acid/AA  <5% >10%Propionic acid/AA<0.1%>0.5%
Some of the main differences, in terms of constituents of the liquid stream exiting from the oxidation reactor, between the ex-propylene and ex-glycerol processes are illustrated in table 1. Naturally, although this is not mentioned in the table, a whole series of oxygen-comprising compounds, alcohols, aldehydes, ketones and other acids, the necessary separation of which is known to a person skilled in the art, is also found in the crude acrylic acid, whether it originates from the ex-propylene process or from the ex-glycerol process.
The acetic acid and the propionic acid cause difficulties for the acrylic acid, in particular because they are not converted during the polymerization process; they are saturated and thus cannot be polymerized. Depending on the polymerization process involved and the applications targeted for the polymer, these impurities may remain in the finished product and risk conferring undesirable corrosive properties on the finished product or be reencountered in the liquid or gaseous discharges generated by the polymerization process and cause organic pollution, which is also undesirable. They therefore have to be removed.
The acetic acid can be removed by distillation in a light fraction, an operation generally denoted topping. However, the reduction in the concentration of acetic acid in the context of the ex-glycerol process results in a consequent loss of acrylic acid in the light fraction, as a result, on the one hand, of the large difference existing between its initial content in the crude acrylic acid and its targeted content in the technical acrylic acid and, on the other hand, of the existence of hydrogen bonds existing between the carboxyl groups of the two molecules. This disadvantage is important economically as the production of a glacial acrylic acid with an acetic acid content of less than 0.1% by weight can only be carried out at the expense of the degree of recovery of the acrylic acid exiting from the oxidation reactor.
As regards the propionic acid, the extremely small difference in volatility existing between this impurity to be removed and the acrylic acid to be purified (of the order of 1° C.) prevents any purification of the acrylic acid by distillation under economically acceptable conditions.
There exists, in the prior art, no process which makes possible the manufacture of compositions sufficiently concentrated in propionic acid of renewable origin to allow them to be used in the conventional applications of the propionic acid obtained with fossil starting materials.